Determination of Oil Removed by Gas via Miscible Displacement in Reservoir Rock

ABSTRACT

Systems, methods, and computer program products can be used for determining the amount of oil removed by a miscible gas flood. One of the methods includes identifying locations of oil within a volume representing a reservoir rock sample. The method includes identifying locations of gas within the volume. The method also includes determining the amount of oil removed based on locations within the volume where oil is either coincident with the gas or is connected to the gas by a continuous oil path.

CLAIM OF PRIORITY

This application claims priority under 35 USC § 119(e) to U.S.Provisional Patent Application Ser. No. 62/720,161, filed on Aug. 21,2018 and entitled “Determination of Oil Removed by Gas via MiscibleDisplacement in Reservoir Rock,” the entire contents of which are herebyincorporated by reference.

BACKGROUND

This disclosure relates to enhanced oil recovery from porous media,especially estimating recoverable quantities at various stages ofenhanced recovery.

The production of oil from a subsurface reservoir can be categorizedinto three phases primary recovery, secondary recovery, and tertiaryrecovery. Primary recovery involves well bores that are drilled into anoil containing reservoir. Oil is recovered due to a driving forceresulting from there being a higher pressure in the reservoir comparedto a pressure in the well bore. Over time, reservoir pressure decreaseswith a concomitant decrease of this driving force to move oil towards tothe well bore. In secondary recovery, brine is pushed into injectionwell bores to increase the reservoir pressure and restore a ‘drivingforce’ to push oil towards extraction wells, (this process is oftenreferred to as water-flooding). The tertiary phase, often referred to asEnhanced Oil Recovery (EOR), attempts to recover oil that remainstrapped in the pore space of the reservoir rock even after primary andsecondary recovery. One common type of EOR method is gas injection (alsocalled gas flooding), in which a gas is injected into the injectionwells. A gas flood can be miscible (gas and oil mix) or immiscible (gasand oil remain separate phases) depending on the gas and oil fluidproperties and reservoir conditions such as temperature and pressure.

Injection of miscible gases are used to maintain a reservoir pressureand are used as solvents to reduce interfacial tension between oil andwater, and thus enhance displacement of hydrocarbons such as oil fromwater. Miscible injection is an economically viable process thatsignificantly increases oil recovery from many different types ofreservoirs. Miscible fluids that are used as solvents to increase oilrecovery include carbon dioxide (CO₂), nitrogen (N), and other gasessuch as liquefied petroleum gas (LPG), propane, methane under highpressure, and methane that is enriched with other hydrocarbons.

SUMMARY

According to an aspect, a computer implemented method for determining anamount of oil recoverable from porous reservoir rock by a miscible gasflood includes identifying locations of oil within a volume of voxelsrepresenting an original state of the porous reservoir rock, identifyinglocations of gas within the volume of voxels representing the originalstate of the porous reservoir rock, and determining voxels within thevolume where oil is either coincident with gas or is connected to gas bya continuous oil path to estimate the amount of oil that is recoverablefrom the porous reservoir rock.

Aspects also include computer program products on non-transitorycomputer readable medium and data processing systems such as computers.

The following are some of the additional features of one or more of theabove aspects.

Identifying locations of oil further includes executing a firstnumerical multiphase flow simulation to obtain predictions of flowbehavior of oil in the presence of a waterflood of the porous reservoirrock; and wherein identifying locations of gas further includesexecuting a second numerical multiphase flow simulation to obtainpredictions of flow behavior for a gas flood of the porous reservoirrock.

The amount of recoverable oil removed by a direct displacement isdetermined based on voxels within the volume where oil is coincidentwith the gas. The amount of recoverable oil removed by extraction andswelling is determined based on voxels within the volume where oil isconnected to the gas by a continuous oil path. The aspect(s) furtherinclude generating a digital representation from data corresponding to athree dimensional imaging of the porous reservoir rock. The aspect(s)further include analyzing the digital representation to determinecharacteristics of a pore space network within the digitalrepresentation. Determining voxels within the volume where oil iscoincident with the gas includes performing a logical AND operation ondata representing the state of the pore space after the waterflood stageand the state of the pore space after the gas flood stage. Determiningvoxels within the volume where oil is connected to the gas by acontinuous oil path includes identifying voxels that have oil and are ina direct displacement and identifying voxels that have connectedhydrocarbon phases to one or more of the voxels within the volume whereoil is coincident with the gas.

The foregoing and other embodiments can each optionally include one ormore of the following features, alone or in combination. The amount ofoil removed by direct displacement may be determined based on locationswithin the volume where oil is coincident with the gas. The amount ofoil removed by extraction and swelling may be determined based onlocations within the volume where oil is connected to the gas by acontinuous oil path.

Other features and advantages will be apparent from the followingdescription, including the drawings, and the claims.

The details of one or more embodiments of the subject matter of thisspecification are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages of thesubject matter will become apparent from the description, the drawings,and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a system for simulation of a waterflood and a gas floodwith an estimation engine to determine an amount of oil removed(recoverable) by a miscible gas flood in a subsurface reservoir.

FIG. 2 depicts a flow chart showing operations for estimatingrecoverable oil by miscible gas flood in the subsurface reservoir.

FIG. 3 depicts a graph showing oil recovery as a function of injectionfor a laboratory study (prior art).

FIG. 4 depicts a graph showing relative permeability from simulation ofa waterflood followed by a gas flood.

FIG. 5 is a flow chart showing operations for estimating recoverableoil.

FIG. 6 is a diagrammatical view of residual oil remaining after awaterflood that is followed by a gas flood.

FIGS. 7A-7D are views of a pore network showing residual oil afterwaterflood, a gas flood, direct displacement and after swelling andextraction.

DESCRIPTION

Referring to FIG. 1 , a system 10 that simulates miscible gas floodingfor determining an amount of oil removed by the miscible gas flood insubsurface reservoir is shown. The system 10 in this implementation isbased on a client-server or cloud based architecture and includes aserver system 12 implemented as a massively parallel computing system 12(stand alone or cloud-based) and a client system 14. The server system12 includes memory 18, a bus system 11, interfaces 20 (e.g., userinterfaces/network interfaces/display or monitor interfaces, etc.) and aprocessing device 24. In memory 18 are a 3D imaging engine 32 thatgenerate a digital representation of a porous reservoir rock formationand an image analysis engine 32 b that determines characteristics of thegeometry of a pore space network in the porous reservoir rock formationfrom analysis of the digital representation of the porous reservoir rockformation. Also in memory is a simulation engine 34 that executes anumerical multiphase flow simulation for various saturation values toobtain predictions of flow behavior including relative permeability as afunction of saturation through the digital representation of the porousreservoir rock formation. In some embodiments, simulating multi-phaseflow behavior occurs through a reservoir rock adjacent a gas or oil well(e.g., drilling rig 37).

The digital representation of the porous reservoir rock formation andthe image analysis conducted by engines 32 a, 32 b, respectively, whichare used to determine the characteristics of the pore space network canbe third party applications that are executed on a different system thanserver 12. The system 10 merely requires the digital representation ofthe formation and the analysis of the pore space network available asinput to the simulation engine 34. One approach to provide the digitalrepresentation 32′ of the reservoir rock sample is to obtain therepresentation 32′, for example, from a 3D imaging of the porousreservoir rock formation that is generated from a micro CT scan ofreservoir rock sample(s).

The memory 18 also stores parameters used by the engine 32, such asgrain surface properties obtained by assigning mineral types 33 a to thegrains to determine the surface properties for each of those mineraltypes, and surface texture and roughness properties. The memory 18 alsostores parameters such as fluid properties 33 b, e.g., fluid density andviscosity of each expected fluid, (e.g., two or more of water, gas, oil)and fluid-fluid interfacial tension properties. The memory 18 alsostores parameters such as chemical composition data 33 c of the fluidsand affinity data 33 d of the fluid components for the particularmineral types. The memory 18 also stores disjoining pressure 33 e foreach mineral type in combination with the fluids, and a selected 33 faging time used by the aging engine 32. In addition, reservoir pressureand temperature data are also stored. The mineral types evaluated can bethose found or expected at the actual site of the reservoir.

The simulation engine 34 includes a module 34 a to set up the reservoirrock sample simulation environment, a module 34 b to perform pore-scalenumerical simulation of a waterflood, a module to perform a gas flood,and a module 34 c to calculate to determine how much oil is produced bythe gas flood, and how much of that produced oil is due to directdisplacement versus extraction and swelling. The system 10 accesses adata repository 38 that stores 2D and/or 3D meshes, coordinate systems,and libraries that can be used in the simulations using any well-knowncomputational technique such as computational fluid dynamics or the socalled Lattice Boltzmann method.

Referring now to FIG. 2 , a process 40 to determine amount of oilremoved by the miscible gas flood in subsurface reservoir is shown. Uponreceiving or retrieving a digital representation of the pore space andgrain space of a reservoir rock sample, the simulation engine 34 sets upinput parameters and conditions 42 to perform a pore-scale numericalsimulation of a waterflood on a first, typically original state of theporous medium (or media). The simulation engine 34 using the set upsimulation environment, performs 43 a a pore-scale numerical simulationof a waterflood. Upon performing the waterflood 43 a, the simulationengine 34 stores 43 b the state of the pore space in memory and performsa gas flood simulation 44 a using the same first, typically originalstate of the porous medium (or media). Upon performing the gas flood 44a, the simulation engine 34 also stores 44 b the state of the pore spacein memory after performance of the gas flood simulation 44 a.

Using results of the simulation of the gas flood, the estimation engine35 that can be part of the simulation engine calculates 46 a an amountof oil that is recovered by the gas flood due to direct displacement andcalculates 46 b the amount that is recovered due to extraction andswelling. The system 10 will be described as using the LBM technique forconducting the waterflood and gas flood simulations, however asmentioned above, any of the various well-known computational techniquesin computational fluid dynamics can be used.

For explanatory purposes a brief description of multiphase simulationsis provided using the “Lattice Boltzmann Method” (LBM) for fluid flowsimulation. Unlike other Computation Fluid Dynamic techniques thatnumerically solve conservation equations of macroscopic properties(i.e., mass, momentum, and energy), the LBM technique models fluids as“particles”, and such particles perform consecutive propagation andcollision processes over a lattice mesh made up of voxels. Voxels can beof different sizes and serve as basic unit area of fluid flow analysis.Voxels are associated with characteristics that describe the state of afluid in a particular region (including, for example, velocity vectorsthat describe the velocity of the fluid in that region). During eachtime-step of a simulation, fluid may move from one voxel to anothervoxel according to the velocity vectors. Collision operators describethe effect of different fluids colliding due to the fluid movement.

The LBM technique can be implemented efficiently on scalable computerplatforms and run with great robustness for time unsteady flows andcomplex boundary conditions. In one example, pore-scale multiphase fluidflow simulation results can be used to understand the fluid displacementmechanisms in reservoir rock. Micro-CT images of reservoir rock can beused to construct the geometry of the porosity that is used as input toeach of the simulations.

The system 10 using the LBM technique determines, relative permeabilityand multi-phase flow through the porous media, which, in general, aredependent on various characteristics of the fluid-fluid-rock system,including rock surface properties, physical properties of each fluid,and flow conditions. One flow condition is the non-dimensional Capillarynumber (Ca) that represents the ratio of viscous forces over capillaryforces, and is usually defined as the average fluid velocity times theviscosity of a reference fluid divided by the fluid-fluid interfacialtension. Another property is the wettability, which represents thepreference of the rock surface for one fluid over another one, andmanifests as a measurable property known as contact angle.

The system 10 when configured for conducting simulations of waterfloodsand gas floods, is first configured for conducting the waterfloodsimulation on the original state of the porous medium, obtainingresults, and subsequently conducting the gas flood simulation on theoriginal state of the porous medium.

For simulating a waterflood, the simulation is set up as a two fluidsimulation, with water being the one fluid that is injected into thepore space and oil being the other fluid. Simulating a waterfloodprocess is essentially simulating a wettability alteration resultingfrom injection of water. The process is set up to be an iterativeprocess where wettability alteration is performed followed by fluidphase redistribution. The process is repeated until the engine 34determines that the wettability alteration converges (as measured byfurther repetitions not significantly changing the results (e.g.,contact angles). The process 40 references an established thresholdvalue that provides an amount by which a contact angle would need tovary to otherwise halt further iterations of wettability alternationsimulations. One such process is disclosed in U.S. patent applicationSer. No. 16/243,285, filed Jan. 9, 2019, entitled “DETERMINING FLUIDFLOW CHARACTERISTICS OF POROUS MEDIUMS,” the entire contents of whichare incorporated herein by reference. The wettability process uses atwo-phase relative permeability (kr) process to determine how easily onefluid, e.g., oil, can be moved through the in the presence of anotherfluid, e.g., water.

Two-phase relative permeability is an important characteristic ofhydrocarbon reservoir rocks and a crucial input to oil and gas reservoirmodeling and simulation activities. One technique to determine two-phaserelative permeability is disclosed in U.S. patent application Ser. No.14/277,909, filed May 15, 2014 entitled “MASS EXCHANGE MODEL FORRELATIVE PERMEABILITY SIMULATION” published as US-2014-0343858-A1 onNov. 20, 2014, the entire contents of which are incorporated herein byreference.

The system 10 when configured for conducting simulations of waterfloodsand gas floods, may also track blobs in the formation. One technique todetermine two-phase relative permeability is disclosed in U.S. patentapplication Ser. No. 14/660,019, filed Mar. 17, 2015 entitled “FLUIDBLOB TRACKING FOR EVALUATION OF MULTIPHASE FLOW SIMULATIONS” publishedas US-2015-0268080-A1 on Sep. 24, 2015, the entire contents of which areincorporated herein by reference.

Upon performance of the waterflood simulation, the gas flood simulationis conducted. The simulation engine 34 is thus configured for conductingthe gas flood simulation from the state of the original porous medium,e.g., the state of pore space prior to the waterflood simulation, withresidual water in the pore space being one of the fluids and hydrocarbon(miscible gas+oil) being the other fluid. The gas flood simulation isconducted using the techniques discussed above, except that thesimulation is configured for the hydrocarbon (combined quantities ofinjected gas and oil) and the residual water.

While exemplary simulation techniques for the waterflood and gas floodsimulations have been described above, it is understood that these areexemplary. Thus in some implementations of estimating a predicted amountof potential oil recovery due to EOR techniques, other simulationtechniques could be used. In addition, in some embodiments, actual fielddata of the state of a pore space can be used in lieu of conductingsimulations.

The recovery of additional oil by a miscible gas flood occur by twopore-scale mechanisms. One mechanism is direct displacement, where oilis directly contacted by the injected gas, and the oil is quicklydissolved and swept away by the flowing gas. Another mechanism isextraction and swelling. Extraction and swelling is a relatively slowerprocess than direct displacement. Extraction and swelling is a processin which oil that is connected by an oil path to the flowing gaseventually reaches the gas and dissolves into it. This process requiresa diffusive mixing of fluids in which gas molecules enter the oil phaseand oil molecules enter the gas phase. Gas phase which contains oilmolecules is also called rich gas.

FIG. 3 shows pore-scale diagrams of these two mechanisms (bottom) and anexample plot of oil recovery during a waterflood followed by a gasflood, taken from https://petrowiki.org/Miscible_flooding (prior art).The graph shows a plot of percentage of Original Oil in Place (OOIP) vs.total pore volumes injected. The schematics at the bottom of FIG. 3illustrate pore-level recovery mechanisms for each of a waterfloodphase, an early miscible (or gas) flood stage, and a late miscible floodstage showing that some of the oil is recovered by displacement bysolvent and some of the oil is recovered by extraction and swelling.

At the end of the waterflood, in the example, the residual oil is adiscontinuous phase that occupies approximately 40% of the pore space.Early in the miscible flood [3.0 to 3.5 total pore volumes (PV)injected], some of this oil has been miscibly displaced by solvent (gas)from the higher-permeability flow path (on the pore scale). However,some oil also has been initially bypassed by solvent (gas). As depictedin the schematic view, late in the waterflood (to 7.0 total PVinjected), part of the locally bypassed oil is subsequently recovered byextraction and swelling that takes place as solvent continues to flowpast the bypassed oil. (The above discussion of FIG. 3 , adapted fromhttps://petrowiki.org/Miscible_flooding (prior art)).

Thus as shown in FIG. 3 , the displacement process of a gas floodinvolves three types of fluid: water, oil, and gas.

As used herein assuming full miscibility of oil and gas, the oil and gashydrocarbons mix and for the purposes of the described simulations, theoil and gas mix will be considered as a single hydrocarbon phase. Thisassumption reduces a three-phase flow simulation to a two-phase flowsimulation of water and hydrocarbon (oil and gas that is treated as asingle phase).

Referring to FIG. 4 , an example of a set of relative permeabilitycurves resulting from a pore-scale numerical simulation of a water flood(solid lines) followed by a gas flood (dashed lines) is shown. In thewater flood procedure (narrow black lines), water displaces oil andwater saturation increases (left to right) until no more oil can bedisplaced. For the gas flood procedure (wide black lines), gas and oilare treated as a single hydrocarbon fluid, and the water saturationdecreases, as gas is injected and the combined gas-oil phase displaceswater.

However, it is not straightforward to know from these results how muchoil was produced during gas injection, because gas and oil are modeledas one fluid phase in the simulation. While the amount of oil producedby the waterflood is clear, it would be valuable to also be able todetermine how much oil is produced by the gas flood, and how much ofthat produced oil is due to direct displacement versus extraction andswelling.

Referring to FIG. 5 , a process 50 to estimate amounts of oil recoveredby the gas flood stage and the amounts of that recovered oil that resultfrom direct displacement versus extraction and swelling is shown. Theprocess 50 is performed by one or more computer systems including memoryand processors, as shown in FIG. 1 .

Fluid data are loaded and the simulation space is configured for awaterflood simulation 51. For example, a digital representation based on3D imaging of the porous reservoir rock is used and connectivityinformation is derived from an analysis of the digital representationthat determines the geometry of the porous space. A mesh (or a latticegrid) is affixed to the digital representation of the pore space. Themesh is comprised of voxels that represent values on the mesh inthree-dimensional space. For each voxel within the mesh, there is adensity for each phase (e.g., a density for water and a density for thehydrocarbon (oil only at this stage)).

A waterflood is performed 52. The waterflood can be a physicalwaterflood of a reservoir rock formation or a numerical simulation ofthe reservoir rock formation or another technique that leaves in thepore space a water background and remaining oil. The state of the porespace after the waterflood stage is determined 54 by any one of physicalimaging or numerical simulation techniques or another technique. Theloaded fluid data are for example from results of an actual waterfloodor a simulation of a waterflood. A simulation of a waterflood wouldinclude a flow simulation through a digital representation of the porespace based on the 3D imaging of the porous reservoir rock. Connectivityinformation would be derived from an analysis of the digitalrepresentation that determines the geometry of the porous space.

From either the flow simulation or the actual waterflood data, a statematrix is built 55 a. The state matrix identifies which cells in thesimulation or actual waterflood data (derived from imaging) areidentified as being part of which phase. Identifying the phase of thecell in the matrix can include comparing the density of the phases inthe voxel for waterflood simulation. For example, if a cell is mostlywater but contains a little oil, then the density of the water in thecell will be much higher than the density of the oil. An Atwood numbercan be calculated for each voxel. The Atwood number can be used todetermine a weighting of how much of each phase is present in the voxel.From these data, oil blobs are identified 55 b. Oil blobs (or blobs) aremade up of voxels that include the target phase, here oil, and are incontact on at least one side. Blobs can be identified by employing afilling algorithm on voxels that are identified as including the targetphase.

Thus, at this junction after the waterflood there is a state of the porespace of blobs and pore space that is saved 55 c in computer memory.These data are processed during the simulation, and the data can includeinformation such as blob identification numbers, blob volumes, bloblocations, and other characteristics. These data can be processed togenerate graphical output (for example, the depiction shown in FIG. 7A(discussed below). The processed data will also indicate where there areblobs of oil that are trapped. Trapped blobs may refer to oil in porespaces that cannot be moved because of capillary forces.

The state of the pore space after the waterflood stage is stored 55 c incomputer memory as a data structure. Any of several data structure typescould be used, or the data could be stored as graph structures, flatfiles, in databases, etc. The state of the pore space comprises locationinformation and whether or not oil is present in each voxel after thewaterflood stage.

A gas flood stage involves loading fluid data and configuring thesimulation space for a gas flood 56 (using the state of the pore spaceat 55 c). The gas flood stage uses a gas, as a solvent, which gas is anyone of the gases commonly used for gas flooding, as explained above(other gases that could be used as the solvent). The gas flood stage isperformed 58. The gas flood can be a physical gas flood of a reservoirrock formation or a numerical simulation of the reservoir rock formationor another technique that leaves a gas background, trapped oil anddisplace-able oil.

The state of the pore space after the gas flood stage is determined 60by any one of physical imaging or numerical simulation techniques oranother technique and is stored in computer memory. These data areprocessed during the gas flood stage simulation, and the data caninclude information such as the blob identification numbers, the blobvolumes, the blob locations, and other characteristics. These data canbe processed to generate graphical output (for example, the depictionshown in FIG. 7B (discussed below). The processed data will alsoindicate where there are blobs of hydrocarbon (oil and gas) that aretrapped. Trapped blobs may refer to hydrocarbon (oil and gas) in porespaces that cannot be moved because of capillary forces. As mentionedthere are only two phases (gas and oil) involved in the gas floodingstage. Because when I read the paragraph, I feel the reader may thinkthere are three phases here (gas, oil and water).

The state of the pore space after the gas flood stage is stored incomputer memory as a data structure and can include building a statematrix 62 a, identify oil blobs 62 b and store 62 c the state of thestate space in memory. Any of several data structure types could beused, or the data could be stored as graph structures, flat files, indatabases, etc. The state of the pore space comprises locationinformation and whether or not hydrocarbon (oil and gas) is present ineach voxel after the gas flood stage.

Subsequent to the analysis performed after the gas flood stage, the datafrom the waterflood stage (the state of the pore space after awaterflood simulation) and the data from the gas flood stage (the stateof the pore space after gas flood simulation) are processed 63 toestimate amounts of oil recovered by the gas flood stage.

The amount of oil recovered from direct displacement can be calculatedby performing 65 a logical “AND” operation on data representing thestate of the pore space after the waterflood stage and on datarepresenting state of the pore space after the gas flood stage. That is,performing the logical operation AND operation involves evaluatingvoxels V_(i,j,k) at the end of waterflood stage that have oil (can beindicated by a logical 1 value in the state matrix), which are “ANDed”with the state of those voxels V_(i,j,k) at the end of gas flood stagethat have the gas. That is, direct displacement involves oil that isremoved by the injected gas at locations where there is both remainingoil (after waterflood) and injected gas (after gas flood) as shown inFIG. 7 c (discussed below). Thus any voxel that has combined oil andgas, i.e., hydrocarbon that voxel V_(i,j,k) will be considered to havecontributed its oil to oil recovery at the direct displacement stage.

The amount of oil can further be estimated by knowing the volume of agiven voxel and for each of the voxel having oil and oil added thevolumes of those voxels to produce a volume estimate. The estimate canbe further refined by taking into consideration the density of thecontents of each voxel, with voxels that have higher densities beingpredominantly water vs. voxels having lower densities beingpredominately hydrocarbons.

The state of the pore space after determining the amount of oilrecovered from direct displacement is stored 66 in computer memory as adata structure. Any of the several data structure types discussed abovecould be used. The state of the pore space comprises locationinformation and whether or not oil in given voxel was displaced.

To estimate amounts of oil recovered by extraction and swelling afterthe gas flood stage the state of the pore space after the gas floodstage (the state of the pore space after gas flooding) is analyzed toidentify 67 any blobs that were recovered and which had connectedhydrocarbon phases that were not contained in the estimated recovery bydirect displacement. Thus a voxel V_(i+n,j+o,k+p) will be considered tohave contributed its oil to oil recovery at the extraction and swellingstage if voxel V_(i+1,j+1,k+1) is connected to V_(i,j,k) and voxelV_(i,j,k) had contributed its oil to oil recovery at the directdisplacement stage, where i+n, j+o, k+p are indices of arbitrarylocations relative to voxel V_(i,j,k).

The amount of oil recovered by extraction and swelling after the gasflood stage can further be estimated by knowing the volume of a givenvoxel and for each of the voxels having oil and gas added the volumes ofthose voxels to produce a volume estimate. The estimate can be furtherrefined by taking into consideration the density of the contents of eachvoxel, with voxels that have higher densities being predominantly watervs. voxels having lower densities being predominately hydrocarbons. Theprocess 50 can then store state space after calculated oil quantity fromextraction and swelling 68.

FIG. 6 shows an illustrative view of a three dimensional (3D)distribution of oil in pore space after a waterflood, as would bepredicted by a pore-scale numerical simulation.

Referring now to FIGS. 7A-7D, these figures are two dimensional viewsthat show rock generally 70 in different stages of oil recovery. Thepore space 70 is present in reservoir rock that is indicated as verydark grey areas 72.

FIG. 7A shows the pore space 70 with water indicated as moderately darkgrey water background 74 and remaining oil indicated as lighter outlinedgrey areas Sa, at the end of a waterflood within the water background72, as determined by any one of physical imaging or numericalsimulation, or another technique.

FIG. 7B shows where in this pore space 70 injected gas (white 76) flowsduring an ensuing gas flood, as determined by physical imaging ornumerical simulation or another technique.

FIG. 7C shows the amount of oil that is remove from the pore space bythe miscible gas flood. Oil is removed by the injected gas at locationswhere there is both remaining oil (indicated as lighter outlined greyareas δS_(α)) and the injected gas (white 76) in the pore space 70.

FIG. 7D shows gas phase and residual oil after swelling and extraction.During the extraction and swelling phase, after the direct displacementphase all remaining oil that was connected to the gas phase by an oilpath is removed from the pore space 70. These locations where remainingoil that was connected to the gas phase are located using a connectivitytest. On the other hand, only isolated oil blobs that do not have an oilpath connection to the gas will remain after extraction and swelling, asshown.

Thus, also referring back to FIG. 7C, the oil blob 75 a was flushed outas it lied directly in the gas path. Bobs 75 b and 75 c were alsoremoved as portions of those blobs also lied directly in the gas path,and remaining portions of those blobs were connected to the portions ofthe blobs and hence connected to or entrained by the gas stream. On theother hand isolated oil blobs 75 d, 75 e remain as these blobs did nothave an oil path connection to the injected gas, and thus remain in thepore space 70 after extraction and swelling.

The amount of oil removed from the pore space by the miscible gas floodis determined by calculating the amount of oil that is recovered bydirect displacement: oil is removed by the injected gas at locationswhere there is both remaining oil ((indicated as lighter outlined greyareas δS_(α))) and injected gas (white 76) as shown in FIG. 7C.

Extraction and swelling thus entails determining after directdisplacement, all remaining oil connected to the gas phase by an oilpath that can be removed from the formation. These locations can befound using a connectivity test (e.g., having common facets or faces ofa voxel, etc.). Conversely, only isolated oil blobs with no oil pathconnection to the gas will remain after extraction and swelling, asindicated in FIG. 7D.

Embodiments of the subject matter and the operations described in thisspecification can be implemented in digital electronic circuitry, or incomputer software, firmware, or hardware, including the structuresdisclosed in this specification and their structural equivalents, or incombinations of one or more of them. Embodiments of the subject matterdescribed in this specification can be implemented as one or morecomputer programs (also referred to as a data processing program) (i.e.,one or more modules of computer program instructions, encoded oncomputer storage medium for execution by, or to control the operationof, data processing apparatus). A computer storage medium can be, or beincluded in, a computer-readable storage device, a computer-readablestorage substrate, a random or serial access memory array or device, ora combination of one or more of them. The computer storage medium canalso be, or be included in, one or more separate physical components ormedia (e.g., multiple CDs, disks, or other storage devices). The subjectmatter may be implemented on computer program instructions stored on anon-transitory computer storage medium.

The operations described in this specification can be implemented asoperations performed by a data processing apparatus on data stored onone or more computer-readable storage devices or received from othersources.

The term “data processing apparatus” encompasses all kinds of apparatus,devices, and machines for processing data, including by way of example:a programmable processor, a computer, a system on a chip, or multipleones, or combinations, of the foregoing. The apparatus can includespecial purpose logic circuitry (e.g., an FPGA (field programmable gatearray) or an ASIC (application specific integrated circuit)). Theapparatus can also include, in addition to hardware, code that createsan execution environment for the computer program in question (e.g.,code that constitutes processor firmware, a protocol stack, a databasemanagement system, an operating system, a cross-platform runtimeenvironment, a virtual machine, or a combination of one or more ofthem). The apparatus and execution environment can realize variousdifferent computing model infrastructures, such as web services,distributed computing and grid computing infrastructures.

A computer program (also known as a program, software, softwareapplication, script, or code) can be written in any form of programminglanguage, including compiled or interpreted languages, declarative orprocedural languages, and it can be deployed in any form, including as astand-alone program or as a module, component, subroutine, object, orother unit suitable for use in a computing environment. A computerprogram may, but need not, correspond to a file in a file system. Aprogram can be stored in a portion of a file that holds other programsor data (e.g., one or more scripts stored in a markup languagedocument), in a single file dedicated to the program in question, or inmultiple coordinated files (e.g., files that store one or more modules,sub programs, or portions of code). A computer program can be deployedto be executed on one computer or on multiple computers that are locatedat one site or distributed across multiple sites and interconnected by acommunication network.

The processes and logic flows described in this specification can beperformed by one or more programmable processors executing one or morecomputer programs to perform actions by operating on input data andgenerating output. The processes and logic flows can also be performedby, and apparatus can also be implemented as, special purpose logiccircuitry (e.g., an FPGA (field programmable gate array) or an ASIC(application specific integrated circuit)).

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor will receive instructions and data from a read only memory ora random access memory or both. The essential elements of a computer area processor for performing actions in accordance with instructions andone or more memory devices for storing instructions and data. Generally,a computer will also include, or be operatively coupled to receive datafrom or transfer data to, or both, one or more mass storage devices forstoring data (e.g., magnetic, magneto optical disks, or optical disks),however, a computer need not have such devices. Moreover, a computer canbe embedded in another device (e.g., a mobile telephone, a personaldigital assistant (PDA), a mobile audio or video player, a game console,a Global Positioning System (GPS) receiver, or a portable storage device(e.g., a universal serial bus (USB) flash drive)). Devices suitable forstoring computer program instructions and data include all forms ofnon-volatile memory, media and memory devices, including by way ofexample, semiconductor memory devices (e.g., EPROM, EEPROM, and flashmemory devices), magnetic disks (e.g., internal hard disks or removabledisks), magneto optical disks, and CD ROM and DVD-ROM disks. Theprocessor and the memory can be supplemented by, or incorporated in,special purpose logic circuitry.

To provide for interaction with a user, embodiments of the subjectmatter described in this specification can be implemented on a computerhaving a display device (e.g., a CRT (cathode ray tube) or LCD (liquidcrystal display) monitor) for displaying information to the user and akeyboard and a pointing device (e.g., a mouse or a trackball) by whichthe user can provide input to the computer. Other kinds of devices canbe used to provide for interaction with a user as well; for example,feedback provided to the user can be any form of sensory feedback (e.g.,visual feedback, auditory feedback, or tactile feedback) and input fromthe user can be received in any form, including acoustic, speech, ortactile input. In addition, a computer can interact with a user bysending documents to and receiving documents from a device that is usedby the user (for example, by sending web pages to a web browser on auser's user device in response to requests received from the webbrowser).

Embodiments of the subject matter described in this specification can beimplemented in a computing system that includes a back end component(e.g., as a data server), or that includes a middleware component (e.g.,an application server), or that includes a front end component (e.g., auser computer having a graphical user interface or a Web browser throughwhich a user can interact with an implementation of the subject matterdescribed in this specification), or any combination of one or more suchback end, middleware, or front end components. The components of thesystem can be interconnected by any form or medium of digital datacommunication (e.g., a communication network). Examples of communicationnetworks include a local area network (“LAN”) and a wide area network(“WAN”), an inter-network (e.g., the Internet), and peer-to-peernetworks (e.g., ad hoc peer-to-peer networks).

The computing system can include users and servers. A user and serverare generally remote from each other and typically interact through acommunication network. The relationship of user and server arises byvirtue of computer programs running on the respective computers andhaving a user-server relationship to each other. In some embodiments, aserver transmits data (e.g., an HTML page) to a user device (e.g., forpurposes of displaying data to and receiving user input from a userinteracting with the user device). Data generated at the user device(e.g., a result of the user interaction) can be received from the userdevice at the server.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of anyinventions or of what may be claimed, but rather as descriptions offeatures specific to particular embodiments of particular inventions.Certain features that are described in this specification in the contextof separate embodiments can also be implemented in combination in asingle embodiment. Conversely, various features that are described inthe context of a single embodiment can also be implemented in multipleembodiments separately or in any suitable subcombination. Moreover,although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the embodiments described above should not be understoodas requiring such separation in all embodiments, and it should beunderstood that the described program components and systems cangenerally be integrated together in a single software product orpackaged into multiple software products.

Thus, particular embodiments of the subject matter have been described.Other embodiments are within the scope of the following claims. Forexample, any of the above techniques that are described with referenceto a pore space can also be performed with reference to or with regardto a physical media, such as a reservoir rock sample. In some cases, theactions recited in the claims can be performed in a different order andstill achieve desirable results. In addition, the processes depicted inthe accompanying figures do not necessarily require the particular ordershown, or sequential order, to achieve desirable results. In certainimplementations, multitasking and parallel processing may beadvantageous.

1. A computer implemented method for determining an amount of oilrecoverable from rock, the method comprising: identifying one or morerepresentations of oil within voxels representing rock; identifying oneor more representations of gas within the voxels representing the rock;and determining one or more of the voxels representing oil and gas toestimate the amount of oil that is recoverable from the rock.
 2. Themethod of claim 1 wherein identifying one or more representations of oilfurther comprises: executing a first numerical multiphase flowsimulation to obtain predictions of flow behavior of oil in the presenceof a waterflood of the rock; and wherein identifying one or morerepresentations of gas further comprises: executing a second numericalmultiphase flow simulation to obtain predictions of flow behavior for agas flood of the rock.
 3. The method of claim 1 wherein the amount ofrecoverable oil is determined based on voxels where oil is coincidentwith the gas.
 4. The method of claim 1 wherein the amount of recoverableoil is determined based on voxels where oil is connected to the gas by acontinuous oil path.
 5. The method of claim 1 further comprising:generating a digital representation from data corresponding to athree-dimensional imaging of the rock.
 6. The method of claim 5 furthercomprising: analyzing the digital representation to determinecharacteristics of a pore space network within the digitalrepresentation.
 7. The method of claim 6 wherein determining one or morevoxels representing oil and gas, comprises: performing a logical ANDoperation on data representing a state of the pore space after awaterflood stage and a state of the pore space after a gas flood stage.8. The method of claim 7 wherein determining one or more of the voxelsrepresenting oil and gas, further comprises: identifying voxels thathave oil and are in a direct displacement; and identifying voxels thathave connected hydrocarbon phases to one or more of the voxelsrepresenting oil and gas.
 9. A system for determining an amount of oilrecoverable from rock, the system comprising: one or more processordevices; memory operatively coupled to the one or more processordevices; and computer storage storing executable program instructions tocause the system to: identify one or more representations of oil withinvoxels representing rock; identify one or more representations of gaswithin the voxels representing the rock; and determine one or more ofthe voxels representing oil and gas to estimate the amount of oil thatis recoverable from the rock.
 10. The system of claim 9 wherein theinstructions to identify one or more representations of oil furthercomprise instructions to: execute a first numerical multiphase flowsimulation to obtain predictions of flow behavior of oil in the presenceof a waterflood of the rock; and wherein instructions to identify one ormore representations of gas further comprise instructions to: execute asecond numerical multiphase flow simulation to obtain predictions offlow behavior for a gas flood of the rock.
 11. The system of claim 9wherein the amount of recoverable oil is determined based on voxelswhere oil is coincident with the gas.
 12. The system of claim 9 whereinthe amount of recoverable oil is determined based on voxels where oil isconnected to the gas by a continuous oil path.
 13. The system of claim9, wherein the instructions further comprise: generate a digitalrepresentation from data corresponding to a three-dimensional imaging ofthe rock; and analyze the digital representation to determinecharacteristics of a pore space network within the digitalrepresentation.
 14. The system of claim 10 wherein determining one ormore of the voxels representing oil and gas, comprises: perform alogical AND operation on data representing a state of the pore spaceafter the waterflood stage and a state of the pore space after the gasflood stage.
 15. The system of claim 14 wherein determining voxels whereoil is connected to the gas by a continuous oil path, further comprises:identify voxels that have oil and are in a direct displacement; andidentify voxels that have connected hydrocarbon phases to one or more ofthe voxels representing oil and gas.
 16. A computer program producttangibly stored on a non-transitory storage device, for determining anamount of oil recoverable from rock, the computer program productcomprising instructions to cause a system to: identify one or morerepresentations of oil within voxels representing rock; identify one ormore representations of gas within the voxels representing the rock; anddetermine one or more of the voxels representing oil and gas to estimatethe amount of oil that is recoverable from the rock.
 17. The computerprogram product of claim 16 wherein the instructions to identify one ormore representations of oil further comprise instructions to: execute afirst numerical multiphase flow simulation to obtain predictions of flowbehavior of oil in the presence of a waterflood of the rock; and whereininstructions to identify one or more representations of gas furthercomprises instructions to: execute a second numerical multiphase flowsimulation to obtain predictions of flow behavior for a gas flood of therock.
 18. The computer program product of claim 16 wherein the amount ofrecoverable oil is determined based on voxels where oil is coincidentwith the gas and where oil is connected to the gas by a continuous oilpath.
 19. The computer program product of claim 16, further comprisinginstructions to: generate a digital representation from datacorresponding to a three-dimensional imaging of the rock; analyze thedigital representation to determine characteristics of a pore spacenetwork within the digital representation; perform a logical ANDoperation on data representing a state of the pore space after awaterflood stage and a state of the pore space after a gas flood stage.20. The computer program product of claim 19 wherein instructions todetermine one or more of the voxels representing oil and gas, furthercomprises instructions to: identify voxels that have oil and are in adirect displacement; and identify voxels that have connected hydrocarbonphases to one or more of the voxels representing oil and gas.
 21. Themethod of claim 1 wherein determining one or more of the voxelsrepresenting oil and gas comprises determining one or more of the voxelsrepresenting oil is either coincident with gas or is connected to gas byan oil path.